1. Field of the Invention
The present invention relates generally to the smelting and refining of copper, and more particularly to the recovery of substantially pure copper from feed material such as secondary copper-bearing scrap metal, as well as ashes, residues, dusts, slimes, and the like, and sulfidic concentrates that are roasted to oxidize the contained metal values. The copper-bearing feed material with which this invention is particularly concerned is contaminated with iron and one or more impurities such as nickel, antimony, tin, arsenic, bismuth and lead, in amounts high enough to make refining of such material impractical by conventional treatment using smelting and electrolytic refining steps.
In conventional practice, secondary copper-bearing source material is fed to a primary smelting furnace where it is smelted under moderately reducing conditions in association with one or more slag-forming components to form a smelted metal product, termed "black copper" (typically 70-80% copper) and a smelter slag product. The black copper is then converted, that is, mixed with a slag-forming agent such as silica sand and vigorously blown with air or oxygen, to form a metallic product known as blister copper (typically 95-97% copper) and a converter slag that contains impurities, including metal values, in an oxidized state. The converter slag is generally recycled to the smelter for recovery of its metal content. The blister copper is then further purified and cast into anodes, and the anodes are electrolyzed in an aqueous electrolyte containing free sulfuric acid and dissolved copper under conditions such that refined copper (over 99.90% copper) is preferentially deposited at the cathode.
In conventional processing of secondary copper-bearing material, it is necessary to limit the amounts of impurity metals, including nickel, tin, and antimony, that are contained in the copper-bearing feed material. If more than a small amount of these impurity metals are present in the copper anodes that are to be electrolytically refined, the impurity metals interfere with the electrolytic refining by causing the quality of the cathode copper deposit and the electrolytic (i.e. energy) efficiency of the refining process to deteriorate. In addition, excessive amounts of antimony or tin form relatively refractory complexes with copper and nickel in the anodes; these complexes collect in the slimes phase during electrolysis of the anodes, and increase the difficulty of treating the slimes for recovery of the metal values contained therein.
Other possible steps for preventing excessive amounts of nickel, antimony and tin from reporting to the blister copper are of little avail. Blowing the impurity metals into the converter slag is futile because, as mentioned above, this slag product is recycled to the feed smelter where the metal values in the slag merely add to the impurity load. Adjusting the conditions under which the feed material is smelted so that most of the impurity metals are driven into the smelter slag helps only to a limited extend because an unacceptable portion of the copper in the feed material also reports to the smelter slag; if the yield of copper in the black copper product is raised to an acceptable level, the impurity metal content of the black copper product rises to an unacceptable level. The slag produced in smelting the feed material usually contains metal values in sufficient quantity to justify a slag cleaning step, which is typically carried out in separate furnace, such as an electric furnace to produce a copper-bearing metal product and a discardable slag product. If excessive amounts of impurity metals are in the slag fed to the cleaning step, the furnace must be operated in a relatively inefficient manner to avoid recovering the impurity metals in the same product as the desired metal values.
Consequently, up to now it has been necessary to limit the amounts of nickel, antimony and tin contained in the copper-bearing feed to a secondary copper refinery employing the conventional sequence of smelting and electrolytic refining steps. Even so, the conditions under which the primary smelting furnace is operated are constrained by the presence of even limited amounts of such impurity metals. Restricting the impurity metal content of the feed disqualifies ever-increasing quantities of copper-bearing material from treatment by this smelting and refining procedure, since the copper is frequently associated with significant amounts of other metals. Thus, a process for recovering copper from secondary feed material which contains nickel, antimony and/or tin in amounts above those which can presently be tolerated in such refineries, and doing so in a manner which is flexible and which reduces the energy consumption per unit of refined copper produced, would be very useful.
2. Description of the Prior Art
In the present invention, some or all of the converter slag is smelted to form anodes that are subsequently electrolyzed. Several patents have recognized the desirability of recovering the metal values from slag produced in non-ferrous metal refining operations, but they do not suggest the improved process of the present invention.
For instance, U.S. Pat. No. 2,820,705 discloses a process for treating slag with a solid carbonaceous reductant under a reducing atmosphere to recover from the slag a first phase comprising a mixture of non-ferrous metal values, and a second phase containing iron and the otherwise metal-depleted slag. The patentee states that the metal mixture can be treated for refining and recovering the constituent metals; one skilled in this art would likely attempt to do so by using conventional means rather than by reintroducing the recovered metal product into a copper refinery circuit in the manner of applicants' invention.
Other processes for recovering non-ferrous metal values from slags are disclosed in U.S. Pat. No. 3,857,700, U.S. Pat. No. 3,984,235, and U.S. Pat. No. 4,032,327, all of which involve matte-based materials and effect the reduction of copper from slag into a molten matte (i.e. sulfur-bearing) phase; and U.S. Pat. No. 4,110,107, in which both copper and iron are reduced from the slag, necessitating subsequent cementation to separate the copper values from the iron.
The electrolytic decomposition of multi-metallic anodes is described in several patents, but none of these patents recognize or suggest applicants' process in which anodes from smelted converter slag are electrolyzed in copper refinery electrolyte.
East German patent publication No. 45,843 discloses a process in which a dross-copper product is treated under reducing conditions to form anodes containing e.g. 90% copper, 3.3% nickel, 1.2% lead, 1% iron, 2.6% tin, 0.45% antimony, and 0.98% sulfur. The anodes are electrolyzed in an acidic copper sulfate electrolyte, which can be conventional copper refinery electrolyte, until the electrolyte becomes turbid. The electrolyte is then boiled for a lengthy period of time under strongly oxidizing conditions to form a white-slime product containing tin oxide and antimony oxide.
In U.S. Pat. No. 1,844,937, copper-nickel matte is roasted and smelted, and the smelted product is then electrolyzed to recover a cathode copper product under conditions such that makeup copper must be supplied to the electrolyte from an external source. The makeup copper is provided as a solution obtained by leaching some of the roasted matte in sulfuric acid. Metal values are recovered from the resulting leach residue by smelting the residue to form anodes containing e.g. about 50% copper and about 50% nickel. These anodes are electrolyzed in a separate acidic electrolyte until the copper content of the electrolyte declines to a residual value of about 5 grams per liter; the residual copper is then cemented from solution by adding metallic nickel-bearing material. This disclosure is distinct from applicants' invention because in the patent the patentee is concerned with electrolysis of the product of smelting roasted matte, and of the residue of an acid leaching step, whereas the present invention does not relate to leaching processes or leach products.
In U.S. Pat. No. 2,023,424, anodes containing copper, zinc, tin and lead are electrolyzed in an aqueous sulfuric acid electrolyte. The resulting zinc sulfate electrolyte is purified of copper by the addition of a suitable amount of zinc oxide and calcium carbonate to precipitate the copper.
U.S. Pat. No. 2,279,900 describes a process for treating secondary zinc- and nickel-bearing cupreous metals. The feed material is melted and cast into anodes which are electrolyzed in sulfuric acid under conditions such that copper dissolves and is deposited at the cathode, while nickel and zinc go into solution in the electrolyte. The patentee describes only conventional techniques for controlling the concentration of dissolved metals in the electrolyte, namely electrolytically precipitating copper using insoluble anodes and then crystallizing sulfates of nickel and zinc from solution. This patent does not describe the treatment of source material containing iron, and one skilled in this art would recognize that the process described in this patent would not work acceptably with source material containing any appreciable amount of iron.
Other patents which describe the electrolysis of multimetallic anodes are U.S. Pat. No. 694,699; U.S. Pat. No. 789,523; U.S. Pat. No. 882,075; and U.S. Pat. No. 1,920,820; and U.S. Pat. No. 2,119,936.